System and method for improving accuracy in a speckle-based image correlation displacement sensor

Abstract

A system and method for improving the accuracy of a speckle-based image correlation displacement sensor provides ultra-high accuracy by ensuring that, in the absence of motion, the speckle image does not vary over time. In one embodiment, the speckle image is stabilized by reducing or compensating for laser diode wavelength changes. Various methods for stabilizing the wavelength include thermoelectric temperature control, measuring and correcting the wavelength by any suitable means, or providing a specific wavelength of light feedback from an external grating. Image stabilization may also be accomplished by monitoring the warm-up process of the system. Once the system is determined to have completed the warm-up process, an indicator is provided to the user to indicate that the system is ready for use. Sensor geometric configurations that reduce or eliminate wavelength-related errors are also disclosed.

1. A speckle-image-correlation optical displacement sensing readhead, the readhead providing speckle pattern images used to determine a relative displacement measurement along at least a first measurement axis relative to a member having a surface that gives rise to speckles when illuminated with coherent radiation, the readhead comprising:
 a coherent light source that emits a light beam that illuminates a portion of the surface to produce a speckle pattern that corresponds to the illuminated portion, the illuminated portion depending on the relative displacement; 
 a readhead optical system having a nominal focal plane, the readhead optical system including an image detector positioned along an optical path of the readhead optical system to receive an image of the speckle pattern; and 
 at least one stabilizing element, wherein the stabilizing element stabilizes the speckle pattern images used to determine the relative displacement measurement with respect to potential changes in the speckle pattern that might otherwise occur due to causes other than the relative displacement along at least a first measurement axis, the at least one stabilizing element comprising a warm-up period monitoring system that provides a signal when the speckle pattern images used to determine a relative displacement measurement are nominally stable. 
 
   
   
     2. The readhead of  claim 1 , wherein the readhead further comprises at least one element used to stabilize a wavelength of the coherent light source, the at least one element comprising at least one of a thermoelectric cooler, and a feedback system that governs operation of the coherent light source such that the wavelength is stabilized. 
   
   
     3. The readhead of  claim 2 , wherein the readhead comprises the feedback system, and the feedback system comprises a grating positioned at an angle relative to a light path including light from the output beam of the coherent light source such that a nominal operating wavelength of light incident on the grating is directed back to reenter the coherent light source. 
   
   
     4. The readhead of  claim 1 , wherein when the readhead is held at a fixed position the warm-up period monitoring system is operable to:
 determine and monitor at least one value comprising at least one of a) an elapsed time since the readhead was turned on, b) a peak correlation value based on two successive images, c) a difference between two successive peak correlation values, d) a correlation quality value based on two successive images, and e) a difference between two successive correlation quality values; and 
 determine when the speckle pattern images used to determine a relative displacement measurement we nominally stable, based on the determined and monitored at least one value. 
 
   
   
     5. The readhead of  claim 1 , wherein the signal comprises at least one of a) a change in state of an indicator light on the readhead and b) an electrical signal output to a host system. 
   
   
     6. The readhead of  claim 1 , wherein the readhead is configured such that the light beam that illuminates a portion of the surface provides normal-incidence illumination on the surface. 
   
   
     7. The readhead of  claim 1 , wherein a prescribed nominal mounting position height of the readhead locates the readhead at an operating height above the surface that gives rise to speckles such that there is an air gap between the readhead and the surface that gives rise to speckles, and such that the nominal focal plane nominally coincides with the surface that gives rise to speckles. 
   
   
     8. The readhead of  claim 1 , wherein the readhead further comprises a mounting position height monitoring system that provides a signal when the readhead is located at a prescribed mounting position height relative to the surface that gives rise to speckles. 
   
   
     9. The readhead of  claim 8 , wherein the prescribed mounting position height relative to the surface that gives rise to speckles locates the readhead such that there is an air gap between the readhead and the surface that gives rise to speckles, and such that the nominal focal plane nominally coincides with the surface that gives rise to speckles. 
   
   
     10. The readhead of  claim 1 , wherein the readhead is configured such that when the readhead is held at a fixed displacement position relative to the surface that gives rise to speckles and the warm-up period monitoring system provides the signal, then the readhead is thereafter operable to provide a set of displacement values such that the members of the set of displacement values do not vary by more than +/−50 nanometers relative to the average value of that set of displacement values, the set of displacement values obtained throughout at least a 12 hour operation period beginning no more than 1 hour after initial turn-on of the readhead. 
   
   
     11. The readhead of  claim 10 , wherein the at least a 12 hour operation period begins no more than 30 minutes after the initial turn-on of the readhead. 
   
   
     12. The readhead of  claim 11 , wherein the at least a 12 hour operation period begins no more than 10 minutes after the initial turn-on of the readhead. 
   
   
     13. The readhead of  claim 10 , wherein the members of the set of displacement values do not vary by more than +/−25 nanometers relative to the average value of the set. 
   
   
     14. The readhead of  claim 10 , wherein the members of the set of displacement values do not vary by more than +/−10 nanometers relative to the average value of the set. 
   
   
     15. A method for operating a speckle-image-correlation optical position transducer readhead, the method comprising:
 (a) providing a readhead usable to measure a relative displacement along at least a first measurement axis relative to a member having an optically diffusing surface, the readhead comprising:
 a coherent light source that emits a light beam that illuminates a portion of the optically diffusing surface, the illuminated portion depending on the relative displacement and producing a scattered light that corresponds to the illuminated portion, and 
 a readhead optical system having a nominal focal plane, the readhead optical system including an image detector positioned along an optical path of the readhead optical system to receive the scattered light, wherein the scattered light creates a speckle intensity pattern on the image detector that corresponds to the illuminated portion of the optically diffusing surface, the speckle intensity pattern comprising a plurality of speckles; 
 
 (b) mounting the readhead at a mounting position height above an optically diffusing surface; and 
 (c) operating the readhead to provide a speckle intensity pattern in a manner that stabilizes the speckle intensity pattern with respect to potential changes in the speckle intensity pattern that might otherwise occur due to causes other than the relative displacement along at least a first measurement axis, wherein operating the readhead in the manner that stabilizes the speckle intensity pattern comprises monitoring 
 the readhead for completion of a system warm-up period and providing an indication when the warm-up is complete. 
 
   
   
     16. The method of  claim 15 , wherein operating the readhead in the manner that stabilizes the speckle intensity pattern comprises operating at least one element to stabilize a wavelength of the coherent light source, the at least one element comprising at least one of a thermoelectric cooler, and a feedback system that governs operation of the coherent light source such that the wavelength is stabilized. 
   
   
     17. The method of  claim 15 , wherein operating the readhead in the manner that stabilizes the speckle intensity pattern comprises operating the readhead at a mounting position height determined such that the nominal focal plane nominally coincides with the optically diffusing surface. 
   
   
     18. The method of  claim 16 , wherein operating at least one element to stabilize a wavelength of the coherent light source comprises operating the feedback system and operating the feedback system comprises utilizing an external grating, wherein the grating is positioned at an angle relative to the output beam of the coherent light source such that a fraction of the wavelength incident on the grating is fed back to the coherent light source. 
   
   
     19. The method of  claim 15 , wherein operating the readhead in the manner that stabilizes the speckle intensity pattern comprises mounting the readhead at mounting position height relative to the optically diffusing surface, wherein the mounting position height locates the readhead relative to the optically diffusing surface such that:
 the prescribed mounting position height defines an air gap between the readhead and the optically diffusing surface; and 
 the mounting position height locates the nominal focal plane of the readhead proximate to the optically diffusing surface such that under a test condition wherein the readhead is operated at the mounting position height and is held at a fixed position relative to the optically diffusing surface and a wavelength of the coherent light source is varied by 0.5 nm, an apparent relative displacement variation of less than 50 nanometers is observed corresponding to the 0.5 nanometer wavelength variation. 
 
   
   
     20. The method of  claim 15 , wherein monitoring the readhead for completion of the system warm-up period and providing an indication when the warm-up is complete comprises the steps of:
 fixing the readhead at an operable position relative to the optically diffusing surface; 
 capturing a first image at a first time and storing it in memory; 
 capturing a second image at a second time after the first time; 
 computing a correlation between the first and second images; 
 determining whether the correlation reaches a threshold value, and if the correlation reaches the threshold value, then providing an indication to one of a user and a host system that the system is ready to measure the relative displacement along at least a first measurement axis, and if the correlation does not reach the threshold value, then repeating the first and second image capture steps, the computing a correlation step, and the determining step. 
 
   
   
     21. The method of  claim 20 , wherein providing the indication comprises activating a light on the readhead. 
   
   
     22. The method of  claim 15 , wherein operating the readhead in the manner that stabilizes the speckle intensity pattern further comprises arranging the readhead such that the light beam that illuminates the portion of the optically diffusing surface provides normal-incidence illumination on the optically diffusing surface. 
   
   
     23. The readhead of  claim 4 , wherein when the warm-up period monitoring system is operable to determine and monitor at least one value comprising at least one of b) and c). 
   
   
     24. The readhead of  claim 4 , wherein when the warm-up period monitoring system is operable to determine and monitor at least one value comprising at least one of d) and e). 
   
   
     25. The readhead of  claim 1 , wherein the readhead has a prescribed mounting position height and the prescribed mounting position height locates the readhead relative to the surface that gives rise to speckles such that:
 the prescribed mounting position height defines an air gap between the readhead and the surface that gives rise to speckles; and 
 the prescribed mounting position height locates the nominal focal plane of the readhead proximate to the surface that gives rise to speckles and such that under a test condition wherein the readhead is mounted at the prescribed mounting position height and is held at a fixed position relative to the surface that gives rise to speckles and a wavelength of the coherent light source is varied by 0.5 nm, an apparent relative displacement variation of less than 50 nanometers is observed corresponding to the 0.5 nanometer wavelength variation. 
 
   
   
     26. The readhead of  claim 25 , wherein the readhead further comprises a mounting position height monitoring system that provides a signal when the readhead is located at the prescribed nominal mounting position height relative to the surface that gives rise to speckles. 
   
   
     27. The readhead of  claim 25 , wherein the prescribed nominal mounting position height locates the readhead relative to the surface that gives rise to speckles such that the surface that gives rise to speckles is positioned beyond the nominal focal plane of the readhead and not more than 0.5 millimeters from the nominal focal plane of the readhead.